Bi-directional cell trafficking between mother and fetus in mouse placenta

In utero position matters for littermate cell transfer in mice: an additional and confounding source with maternal microchimerism

Introduction

Feto-maternal cell transfer during pregnancy is called microchimerism (Mc). Its persistence in respective hosts is increasingly studied as to its potential role in immune tolerance, autoimmunity, cancer, and degenerative diseases. Murine models with transgenic reporter genes, heterozygously carried by the mother, allow maternal Mc tracking in wild-type (WT) offspring. However, as gestation in mice is multi-embryonic, an exchange of cells between fetuses carrying the same reporter gene as their mother and negative WT littermate, named littermate Mc (LMc), can occur and be confounded with the maternal source. We propose here to evaluate LMc contribution in mice.

Methods

To avoid the maternal confounding source of Mc, transgenic males, heterozygous for a reporter gene, here, the human leukocyte antigen DRB1*04 (DR4+/-), were crossed with WT females (DR4-/-). DR4+/- LMc was specifically quantified by HLA-DR4 quantitative PCR, i) in utero in main organs from 15 DR4-/- fetuses from three litters of 11, nine, and five; and ii) after birth in two litters of eight pups: in two DR4-/- stillborns and four DR4-/- adult mice.

Results

At embryonic stages, DR4-/- fetuses having one or two nearby DR4+/- littermates in the same uterine horn were almost seven times more frequently positive for DR4- microchimerism in their organs (p = 0.01) and had quantitatively more LMc (p = 0.009) than those without nearby DR4+/- littermates. Furthermore, LMc persists at birth and into adulthood with interindividual heterogeneity.

Conclusions

This study identifies heterogeneity for LMc acquisition according to in utero position and different interpretation of previously published results on maternal Mc in mice.

Maternal dendritic cells influence fetal allograft response following murine in-utero hematopoietic stem cell transplantation

BACKGROUND

Intrauterine hematopoietic stem cell transplantation (IUT), potentially curative in congenital haematological disease, is often inhibited by deleterious immune responses to donor cells resulting in subtherapeutic donor cell chimerism (DCC). Microchimerism of maternal immune cells (MMc) trafficked into transplanted recipients across the placenta may directly influence donor-specific alloresponsiveness, limiting DCC. We hypothesized that dendritic cells (DC) among trafficked MMc influence the development of tolerogenic or immunogenic responses towards donor cells, and investigated if maternal DC-depletion reduced recipient alloresponsiveness and enhanced DCC.

METHODS

Using transgenic CD11c.DTR (C57BL/6) female mice enabled transient maternal DC-depletion with a single dose of diphtheria toxin (DT). CD11c.DTR females and BALB/c males were cross-mated, producing hybrid pups. IUT was performed at E14 following maternal DT administration 24 h prior. Bone marrow-derived mononuclear cells were transplanted, obtained from semi-allogenic BALB/c (paternal-derived; pIUT), C57BL/6 (maternal-derived; mIUT), or fully allogenic (aIUT) C3H donor mice. Recipient F1 pups were analyzed for DCC, while maternal and IUT-recipient immune cell profile and reactivity were examined via mixed lymphocyte reactivity functional assays. T- and B-cell receptor repertoire diversity in maternal and recipient cells were examined following donor cell exposure.

RESULTS

DCC was highest and MMc was lowest following pIUT. In contrast, aIUT recipients had the lowest DCC and the highest MMc. In groups that were not DC-depleted, maternal cells trafficked post-IUT displayed reduced TCR & BCR clonotype diversity, while clonotype diversity was restored when dams were DC-depleted. Additionally, recipients displayed increased expression of regulatory T-cells and immune-inhibitory proteins, with reduced proinflammatory cytokine and donor-specific antibody production. DC-depletion did not impact initial donor chimerism. Postnatal transplantation without immunosuppression of paternal donor cells did not increase DCC in pIUT recipients; however there were no donor-specific antibody production or immune cell changes.

CONCLUSIONS

Though maternal DC depletion did not improve DCC, we show for the first time that MMc influences donor-specific alloresponsiveness, possibly by expanding alloreactive clonotypes, and depleting maternal DC promotes and maintains acquired tolerance to donor cells independent of DCC, presenting a novel approach to enhancing donor cell tolerance following IUT. This may have value when planning repeat HSC transplantations to treat haemoglobinopathies.

A Framework for Understanding Maternal Immunity

This is an alternative and controversial framing of the data relevant to maternal immunity. It argues for a departure from classical theory to view, interrogate and interpret existing data.

Emergent roles of maternal microchimerism in postnatal development

Maternal microchimerism (MMc) is the phenomenon that a low number of cells from the mother persists within her progeny. Despite their regular presence in mammalian pregnancies, the overall cell type repertoire and roles of maternal cells, especially after birth, remain unclear. By using transgenic mouse strains and human umbilical blood samples, recent studies have for the first time characterized and quantified MMc cell type repertoires in offspring, identified the cross-generational influence on fetal immunity, and determined possible factors that affect their presence in offspring. This review summarizes new findings, especially on the maternal cell type repertoires and their potential role in utero, in postnatal life, and long after birth.

Whole-embryonic identification of maternal microchimeric cell types in mouse using single-cell RNA sequencing

Even though the mother and the fetus of placental mammals are immunologically non-self with respect to one other, mutual exchange of small numbers of cells between them is known to occur. Maternal cells entering the fetus, called maternal microchimeric cells (MMc cells), are thought to be involved in different physiological phenomena, such as establishing immune tolerance, tissue repair, and the pathogenesis or deterioration of some inflammatory diseases and congenital malformations. While specific MMc cell types have been reported as associated with these phenomena, the contribution of MMc cells to these different outcomes remains unknown. As one possibility, we hypothesized that different embryos have differing repertoires of MMc cell types, leading to or biasing embryos toward different fates. To date, no studies have succeeded in identifying the MMc cell type repertoire of a single embryo. Accordingly, here, we isolated MMc cells from whole mouse embryos, determined their types, and analyzed their MMc cell type variability. By combining our previously established, whole-embryonic MMc isolation method with single-cell RNA sequencing, we successfully estimated the cell type repertoires of MMc cells isolated from 26 mouse embryos. The majority of MMc cells were immune-related cells, such as myeloid cells and granulocytes. We also detected stem cell-like MMc cells expressing proliferation marker genes and terminally differentiated cells. As hypothesized, we noted statistically significant inter-individual variation in the proportion of immune-related cells in the different embryos. We here successfully estimated MMc cell types in individual whole mouse embryos. The proportion of immune-related cells significantly differed among the individual embryos, suggesting that the variations are one of the potential mechanisms underlying the differing MMc-related physiological phenomena in offspring. These findings provide insight into cell-level epigenetics by maternal cells.

Postnatal depletion of maternal cells biases T lymphocytes and natural killer cells' profiles toward early activation in the spleen

The maternal cells transferred into the fetus during gestation persist long after birth in the progeny. These maternal cells have been hypothesized to promote the maturation of the fetal immune system in utero but there are still significant gaps in our knowledge of their potential roles after birth. To provide insights into these maternal cells' postnatal functional roles, we set up a transgenic mouse model to specifically eliminate maternal cells in the neonates by diphtheria toxin injection and confirmed significant depletion in the spleens. We then performed immunophenotyping of the spleens of two-week-old pups by mass cytometry to pinpoint the immune profile differences driven by the depletion of maternal cells in early postnatal life. We observed a heightened expression of markers related to activation and maturation in some natural killer and T cell populations. We hypothesize these results to indicate a potential postnatal regulation of lymphocytic responses by maternal cells. Together, our findings highlight an immunological influence of maternal microchimeric cells postnatally, possibly protecting against adverse hypersensitivity reactions of the neonate at a crucial time of new encounters with self and environmental antigens.

Feto-maternal microchimerism: Memories from pregnancy

There is a bidirectional transplacental cell trafficking between mother and fetus during pregnancy in placental mammals. The presence and persistence of fetal cells in maternal tissues are known as fetal microchimerism (FMc). FMc has high multilineage potential with a great ability to differentiate and functionally integrate into maternal tissue. FMc has been found in various maternal tissues in animal models and humans. Its permanence in the maternal body up to decades after delivery suggests it might play an essential role in maternal pathophysiology. Studying the presence, localization, and characteristics of FMc in maternal tissues is key to understanding its impact on the woman's body. Here we comprehensively review the existence of FMc in different species and organs and tissues, aiming to better characterize their possible role in human health and disease. We also highlight several methodological considerations that would optimize the detection, quantification, and functional determination of FMc.

Whole embryonic detection of maternal microchimeric cells highlights significant differences in their numbers among individuals

During pregnancy in placental mammals, small numbers of maternal cells (maternal microchimeric cells, or MMc cells) migrate into the fetus and persist decades, or perhaps for the rest of their lives, and higher frequencies of MMc cells are reported to correlate with variety of phenomena, such as immune tolerance, tissue repair, and autoimmune diseases. While detection of these MMc cells is considered in all pregnancies, their frequency differs largely according to tissue type and disease cases, and it remains unclear whether the number of MMc cells differs significantly among embryos in normal pregnancies. Here, for the first time, we developed a whole embryonic detection method for MMc cells using transgenic mice and counted live MMc cells in each individual embryo. Using this technique, we found that the number of MMc cells was comparable in most of the analyzed embryos; however, around 500 times higher number of MMc cells was detected in one embryo at the latest stage. This result suggests that the number of MMc cells could largely differ in rare cases with unknown underlying mechanisms. Our methodology provides a basis for testing differences in the numbers of MMc cells among individual embryos and for analyzing differences in MMc cell type repertoires in future studies. These data could provide a hint toward understanding the mechanisms underlying the variety of apparently inconsistent MMc-related phenomena.

Addressing microchimerism in pregnancy by ex vivo human placenta perfusion

The physical connection of mother and offspring during pregnancy allows the bi-directional exchange of a small number of cells through the placenta. These cells, which can persist long-term in the recipient individual are genetically foreign to it and therefore fulfill the principle of microchimerism. Over the last years, pioneer research on microchimeric cells revealed their role in immune adaptation during pregnancy and priming of tolerogenic responses in the progeny. However, the mechanisms involved in cell transfer across the placenta barrier remain poorly investigated. In this review, we summarize the evidence of fetomaternal microchimerism, propose a mechanism for cell trafficking through the placenta and discuss the different models and techniques available for its analysis. Likewise, we aim to generate interest in the use of ex vivo placenta perfusion to investigate microchimerism in physiological and pathological settings.

The Role of Neutrophils and Their Extracellular Traps in the Synergy of Pre-eclampsia and HIV Infection

PURPOSE OF THE REVIEW

In our innate immune system, neutrophils are the first cells to sense signals of infection and to proceed to kill the invading pathogen. This is mediated by their production of neutrophil extracellular traps (NETS) to entrap pathogenic micro-organisms, preventing their amplification and dissemination. Pre-eclampsia (PE) is the leading cause of global maternal mortality, yet to date, there is no cure nor a gold-standard diagnostic strategy. The purpose of this review is to discover the role of neutrophils in PE as early identification markers. Additionally, this review aims to explore the role of neutrophils in HIV-infected pregnancies with PE as a source of synergy.

RECENT FINDINGS

Recent findings demonstrate an elevation of neutrophils and neutrophil extracellular traps (NETs) in PE placentae. This is due to their activation by excessive release of syncytiotrophoblast microparticles (STBM). There is also an elevation of NETs in HIV-infected placentae-where histone H3 entraps HIV by binding to its glycoprotein envelope. Additionally, histones H1 and H2A inhibit HIV infection. It is interesting to note that women with both PE and HIV infection have supressed NETs. This review focuses on the role of neutrophils in the synergy of PE and HIV infection. It is plausible that the deregulation of NETs in the synergy of pre-eclamptic HIV-infected women is strategic for the entrapment of the HIV-1 virus. Finally, it is plausible that neutrophils and NETS may act as early biomarkers of PE development. Graphical abstract.

Assessment of microchimerism following somatic cell nuclear transfer and natural pregnancies in goats

Microchimerism is defined as the presence of a small population of cells or DNA in 1 organism originated from a genetically different organism. It is well established that this phenomenon occurs in humans and mice as cells are exchanged between mother and fetus during gestation. Currently, no information is available about the presence of maternal microchimerism in goats, and the only published study is limited to an evaluation of fetal and fetal-fetal microchimerism in blood samples following natural breeding. In order to determine whether bidirectional fetal-maternal cell or DNA trafficking occurs in goats, we assessed: 1) fetal microchimerism in surrogates that gave birth to somatic cell nuclear transfer (SCNT)-derived transgenic offspring (n = 4), 2) maternal microchimerism following natural breeding of SCNT-derived transgenic does with a nontransgenic buck (n = 4), and 3) fetal-fetal microchimerism in nontransgenic twins of transgenic offspring (n = 3). Neomycin-resistance gene (NEO) gene was selected as the marker to detect the presence of the αMHC-TGF-β1-Neo transgene in kidney, liver, lung, lymph node, and spleen. We found no detectable maternal or fetal-fetal microchimerism in the investigated tissues of nontransgenic offspring. However, fetal microchimerism was detected in lymph node tissue of one of the surrogate dams carrying a SCNT pregnancy. These results indicate occurrence of cell trafficking from fetus to mother during SCNT pregnancies. The findings of this study have direct implications on the use and disposal of nontransgenic surrogates and nontransgenic offspring.

Inherited nongenetic influences on the gut microbiome and immune system

The gut microbiome and the immune system codevelop around the time of birth, well after genetic information has been passed from the parents to the offspring. Each of these "organ systems" displays plasticity. The immune system can mount highly specific adaptive responses to newly encountered antigens, and the gut microbiota is affected by changes in the environment. Despite this plasticity, there is a growing appreciation that these organ systems, once established, are remarkably stable. In health, the immune system rapidly mounts responses to infections, and once cleared, resolves inflammatory responses to return to homeostasis. However, a skewed immune system, such as seen in allergy, does not easily return to homeostasis. Allergic responses are often seen to multiple antigens. Likewise, a dysbiotic gut microbiota is seen in multiple diseases. Attempts to reset the gut microbiota as a therapy for disease have met with varied success. Therefore, how these codeveloping "organ systems" become established is a central question relevant to our overall health. Recent observations suggest that maternal factors encountered both in utero and after birth can directly or indirectly impact the development of the offspring's gut microbiome and immune system. Here, we discuss how these nongenetic maternal influences can have long-term effects on the progeny's health.

Role of Regulatory T Cells in Noninherited Maternal Antigen-Related Tolerance in Cord Blood: An in Vitro Study

Cord blood (CB) is an alternative stem cell source for allogeneic hematopoietic stem cell transplantation (HSCT). The unique advantages of using CB as a stem cell source are a degree of permissibility for HLA mismatch, rapid availability, and relatively risk-free cell collection. Because HLA is highly polymorphic and population-specific, optimal HLA-matched unrelated donors or cord blood units (CBUs) might not be available. In view of the possibility that matched CBUs that include noninherited maternal antigens (NIMAs) might contain acceptable HLA mismatches, we attempted to determine the degree of alloreactivity of CB mononuclear cells (MNCs) on stimulation by the maternal, paternal, and unrelated stimulator cells. Suppression of T cell proliferation, cytotoxicity, and a cytokine profile indicating suppressed Th1 and elevated IL-10 and TGF-β1 responses were observed in the mixed lymphocyte reaction in response to NIMAs. The increases in IL-10 and TGF-β1 production may be due to the Th2 response and/or regulatory T cells (Tregs). The reduced IL-10 and TGF-β1 production after CD25 depletion could have been due to removal of Tregs from the CB cells. Thus, Tregs appear to play an important role in the CB MNC response to NIMAs, possibly due to the induction of IL-10 and TGF-β1. We hope that our work can provide some evidence of the beneficial effect of NIMAs.

Prenatal Immune and Endocrine Modulators of Offspring's Brain Development and Cognitive Functions Later in Life

Milestones of brain development in mammals are completed before birth, which provide the prerequisite for cognitive and intellectual performances of the offspring. Prenatal challenges, such as maternal stress experience or infections, have been linked to impaired cognitive development, poor intellectual performances as well as neurodevelopmental and psychiatric disorders in the offspring later in life. Fetal microglial cells may be the target of such challenges and could be functionally modified by maternal markers. Maternal markers can cross the placenta and reach the fetus, a phenomenon commonly referred to as “vertical transfer.” These maternal markers include hormones, such as glucocorticoids, and also maternal immune cells and cytokines, all of which can be altered in response to prenatal challenges. Whilst it is difficult to discriminate between the maternal or fetal origin of glucocorticoids and cytokines in the offspring, immune cells of maternal origin—although low in frequency—can be clearly set apart from offspring's cells in the fetal and adult brain. To date, insights into the functional role of these cells are limited, but it is emergingly recognized that these maternal microchimeric cells may affect fetal brain development, as well as post-natal cognitive performances and behavior. Moreover, the inheritance of vertically transferred cells across generations has been proposed, yielding to the presence of a microchiome in individuals. Hence, it will be one of the scientific challenges in the field of neuroimmunology to identify the functional role of maternal microchimeric cells as well as the brain microchiome. Maternal microchimeric cells, along with hormones and cytokines, may induce epigenetic changes in the fetal brain. Recent data underpin that brain development in response to prenatal stress challenges can be altered across several generations, independent of a genetic predisposition, supporting an epigenetic inheritance. We here discuss how fetal brain development and offspring's cognitive functions later in life is modulated in the turnstile of prenatal challenges by introducing novel and recently emerging pathway, involving maternal hormones and immune markers.

Maintaining T cell tolerance of alloantigens: Lessons from animal studies

Achieving host immune tolerance of allogeneic transplants represents the ultimate challenge in clinical transplantation. It has become clear that different cells and mechanisms participate in acquisition versus maintenance of allograft tolerance. Indeed, manipulations which prevent tolerance induction often fail to abrogate tolerance once it has been established. Hence, elucidation of the immunological mechanisms underlying maintenance of T cell tolerance to alloantigens is essential for the development of novel interventions that preserve a robust and long lasting state of allograft tolerance that relies on T cell deletion in addition to intra-graft suppression of inflammatory immune responses. In this review, we discuss some essential elements of the mechanisms involved in the maintenance of naturally occurring or experimentally induced allograft tolerance, including the newly described role of antigen cross-dressing mediated by extracellular vesicles.

Peptide Sharing Between Viruses and DLX Proteins: A Potential Cross-Reactivity Pathway to Neuropsychiatric Disorders

The present study seeks to determine potential associations between viral infections and neuropsychiatric diseases. To address this issue, we investigated the peptide commonalities between viruses that have been related to psychiatric and neurological disorders—such as rubella, human immunodeficiency virus, and herpesviruses—and human distal-less homeobox (DLX) proteins expressed in developing brain—namely, DLX1, DLX2, DLX5, and DLX6. Peptide matching analyses revealed a high degree of pentapeptide sharing. From an immunological perspective, this overlap is relevant because pentapeptides are endowed with immunogenicity and antigenicity—that is, they are immune determinants. Moreover, infection-induced immune cross-reactions might have functional, spatial, and temporal implications related to the functions and expression patterns of DLX1 and DLX5 in the fetal and adult human brain. In sum, our data support the hypothesis that viral infections may be linked to neuropsychiatric diseases through autoimmune cross-reactions caused by molecular mimicry between viral proteins and brain-specific DLX self-antigens.

Maternal Immunization: New Perspectives on Its Application Against Non-Infectious Related Diseases in Newborns

The continuous evolution in preventive medicine has anointed vaccination a versatile, human-health improving tool, which has led to a steady decline in deaths in the developing world. Maternal immunization represents an incisive step forward for the field of vaccination as it provides protection against various life-threatening diseases in pregnant women and their children. A number of studies to improve prevention rates and expand protection against the largest possible number of infections are still in progress. The complex unicity of the mother-infant interaction, both during and after pregnancy and which involves immune system cells and molecules, is an able partner in the success of maternal immunization, as intended thus far. Interestingly, new studies have shed light on the versatility of maternal immunization in protecting infants from non-infectious related diseases, such as allergy, asthma and congenital metabolic disorders. However, barely any attempt at applying maternal immunization to the prevention of childhood cancer has been made. The most promising study reported in this new field is a recent proof of concept on the efficacy of maternal immunization in protecting cancer-prone offspring against mammary tumor progression. New investigations into the possibility of exploiting maternal immunization to prevent the onset and/or progression of neuroblastoma, one of the most common childhood malignancies, are therefore justified. Maternal immunization is presented in a new guise in this review. Attention will be focused on its versatility and potential applications in preventing tumor progression in neuroblastoma-prone offspring.

Advancing the detection of maternal haematopoietic microchimeric cells in fetal immune organs in mice by flow cytometry

Maternal microchimerism, which occurs naturally during gestation in hemochorial placental mammals upon transplacental migration of maternal cells into the fetus, is suggested to significantly influence the fetal immune system. In our previous publication, we explored the sensitivity of quantitative polymerase chain reaction and flow cytometry to detect cellular microchimerism. With that purpose, we created mixed cells suspensions in vitro containing reciprocal frequencies of wild type cells and cells positive for enhanced green fluorescent protein or CD45.1⁺, respectively. Here, we now introduce the H-2 complex, which defines the major histocompatibility complex in mice and is homologous to HLA in human, as an additional target to detect maternal microchimerism among fetal haploidentical cells. We envision that this advanced approach to detect maternal microchimeric cells by flow cytometry facilitates the pursuit of phenotypic, gene expression and functional analysis of microchimeric cells in future studies.

Cigarette smoke exposure during pregnancy alters fetomaternal cell trafficking leading to retention of microchimeric cells in the maternal lung

Cigarette smoke exposure causes chronic oxidative lung damage. During pregnancy, fetal microchimeric cells traffic to the mother. Their numbers are increased at the site of acute injury. We hypothesized that milder chronic diffuse smoke injury would attract fetal cells to maternal lungs. We used a green-fluorescent-protein (GFP) mouse model to study the effects of cigarette smoke exposure on fetomaternal cell trafficking. Wild-type female mice were exposed to cigarette smoke for about 4 weeks and bred with homozygote GFP males. Cigarette smoke exposure continued until lungs were harvested and analyzed. Exposure to cigarette smoke led to macrophage accumulation in the maternal lung and significantly lower fetal weights. Cigarette smoke exposure influenced fetomaternal cell trafficking. It was associated with retention of GFP-positive fetal cells in the maternal lung and a significant reduction of fetal cells in maternal livers at gestational day 18, when fetomaternal cell trafficking peaks in the mouse model. Cells quickly clear postpartum, leaving only a few, difficult to detect, persisting microchimeric cells behind. In our study, we confirmed the postpartum clearance of cells in the maternal lungs, with no significant difference in both groups. We conclude that in the mouse model, cigarette smoke exposure during pregnancy leads to a retention of fetal microchimeric cells in the maternal lung, the site of injury. Further studies will be needed to elucidate the effect of cigarette smoke exposure on the phenotypic characteristics and function of these fetal microchimeric cells, and confirm its course in cigarette smoke exposure in humans.

Assessment of fetal cell chimerism in transgenic pig lines generated by Sleeping beauty transposition

Human cells migrate between mother and fetus during pregnancy and persist in the respective host for long-term after birth. Fetal microchimerism occurs also in twins sharing a common placenta or chorion. Whether microchimerism occurs in multiparous mammals such as the domestic pig, where fetuses have separate placentas and chorions, is not well understood. Here, we assessed cell chimerism in litters of wild-type sows inseminated with semen of transposon transgenic boars. Segregation of three independent monomeric transposons ensured an excess of transgenic over non-transgenic offspring in every litter. Transgenic siblings (n = 35) showed robust ubiquitous expression of the reporter transposon encoding a fluorescent protein, and provided an unique resource to assess a potential cell trafficking to non-transgenic littermates (n = 7) or mothers (n = 4). Sensitive flow cytometry, fluorescence microscopy, and real-time PCR provided no evidence for microchimerism in porcine littermates, or piglets and their mothers in both blood and solid organs. These data indicate that the epitheliochorial structure of the porcine placenta effectively prevents cellular exchange during gestation.

Clinical implications of maternal-fetal cellular trafficking

Maternal-fetal cellular trafficking (MFCT) is the bidirectional passage of cells that results in the presence of fetal cells in the mother and maternal cells in the fetus. This naturally occurring biological phenomenon has been implicated in the pathogenesis of autoimmune diseases in both mothers and children. However, MFCT may also have beneficial consequences in establishing and maintaining maternal-fetal tolerance and may have long-term consequences for transplantation tolerance. There is also evidence that trafficking is altered during pregnancy complications and fetal intervention. An improved understanding of cellular trafficking during pregnancy will lead to progress in multiple fields including autoimmunity, transplantation, and fetal surgery.

Increased maternal microchimerism after open fetal surgery

Maternal-fetal cellular trafficking (MFCT) during pregnancy leads to the presence of maternal cells in the fetus and of fetal cells in the mother. Since this process may be altered in cases of pregnancy complications, we asked whether open fetal surgery leads to changes in microchimerism levels. We analyzed maternal and fetal microchimerism in fetuses who underwent open fetal surgery for repair of spina bifida and compared their levels to patients who had postnatal repair and to healthy controls. We found that maternal microchimerism levels were increased in patients who had open fetal surgery compared with controls. In contrast, patients who had fetal intervention at the time of delivery did not demonstrate increased microchimerism. These results suggest that open fetal surgery may alter trafficking. Given the importance of MFCT in maternal-fetal tolerance, we discuss potential implications for the field of preterm labor and transplantation tolerance.

The natural history of fetal cells in postpartum murine maternal lung and bone marrow: a two-stage phenomenon

During pregnancy, fetal cells cross into the maternal organs where they reside postpartum. Evidence from multiple laboratories suggests that these microchimeric fetal cells contribute to maternal tissue repair after injury. In mouse models, most injury experiments are performed during pregnancy; however, in a clinical setting most injuries or diseases occur postpartum. Therefore, experiments using animal models should be designed to address questions in the time period following delivery. In order to provide a baseline for such experiments, we analyzed the natural history of fetal cells in the postpartum maternal organs. Female C57BL/6J mice were mated to males homozygous for the enhanced green fluorescent protein gene. Fetal cells in the maternal lungs and bone marrow were identified by their green fluorescence using in a high-speed flow cytometer and their counts were compared between the lung and bone marrow. Spearman correlation analysis was used to identify relationships between the duration of time postpartum and the cell counts and ratio of live and dead cells. Our results show that fetal cells persist in these organs until at least three months postpartum in healthy female mice. We show a two-stage decline, with an initial two and a half-week rapid clearance followed by a trend of gradual decrease. Additionally, an increase in the ratio of live to dead cells within the lung over time suggests that these cells may replicate in vivo. The results presented here will inform the design of future experiments and may have implications for women's health.

A mouse model for fetal maternal stem cell transfer during ischemic cardiac injury

Fetal cells enter the maternal circulation during pregnancies and can persist in blood and tissues for decades, creating a state of physiologic microchimerism. Microchimerism refers to acquisition of cells from another individual and can be due to bidirectional cell traffic between mother and fetus during pregnancy. Peripartum cardiomyopathy, a rare cardiac disorder associated with high mortality rates has the highest recovery rate amongst all etiologies of heart failure although the reason is unknown. Collectively, these observations led us to hypothesize that fetal cells enter the maternal circulation and may be recruited to the sites of myocardial disease or injury. The ability to genetically modify mice makes them an ideal system for studying the phenomenon of microchimerism in cardiac disease. Described here is a mouse model for ischemic cardiac injury during pregnancy designed to study microchimerism. Wild-type virgin female mice mated with eGFP male mice underwent ligation of the left anterior descending artery to induce a myocardial infarction at gestation day 12. We demonstrate the selective homing of eGFP cells to the site of cardiac injury without such homing to noninjured tissues suggesting the presence of precise signals sensed by fetal cells enabling them to target diseased myocardium specifically.

Immunological tolerance during fetal development: from mouse to man

The development of the adaptive immune system has been studied in the mouse primarily because it is easier to access fetal tissues and because there exists a rich array of probes for analysis of various components of the immune system. While much has been learned from this exercise, it is also clear that different species show substantial temporal variation in the development of the immune system during early life. In mice, for instance, mature α/β T cells first appear in the periphery during the final stages of fetal gestation and only increase in number after birth (Friedberg and Weissman, 1974); in humans, on the other hand, the first mature α/β T cells are seen in peripheral tissues at 10-12 gestational weeks (g.w.) and are circulating in significant numbers by the end of the second trimester (Ceppellini et al., 1971; Haynes et al., 1988; Hayward and Ezer, 1974; Kay et al., 1970). Although the functional implications of these differences remain unclear, it is likely that there are significant biological consequences associated with the relatively early development of the peripheral adaptive immune system in humans, for example, with respect to the development of peripheral tolerance as well as to the response to antigens that might cross the placenta from the mother (e.g., cells bearing noninherited maternal alloantigens, infectious agents, food antigens, and the like). Here, we will review studies of immune system ontogeny in the mouse and in humans, and then focus on the possible functional roles of fetal T cell populations during development and later in life in humans.

Impact of mouse pregnancy on thymic T lymphocyte subsets

It has been reported that fetal lymphoid progenitor cells are acquired during gestation and are able to develop in the maternal mouse thymus into functional T cells. Moreover, previous pregnancies increase the number of fetal cells in the mother. In the present study, we investigated whether mouse pregnancy induces changes in T lymphocyte subsets in the maternal thymus. We determined the T lymphocyte subsets in two allogeneic cross-breedings, namely CBA/J × BALB/c (normal) and CBA/J × DBA/2 (abortion prone), and investigated the effects of the age and parity of the female, as well as pregnancy outcome, on thymocyte populations. In addition, hormonal effects were evaluated in a syngeneic combination (CBA/J × CBA/J). We found that during pregnancy both hormonal and allogeneic stimuli induced a reduction in the CD4+CD8+ subset with an increase in the CD4+CD8– population. Only young females of the normal combination exhibited an increase in the CD4–CD8+ population. All young mice showed an increase in CD4+CD25+FoxP3+ T cells. Interestingly, the γδT thymus pool was increased in all females of the normal allogeneic pregnancy only, suggesting the participation of this pool in the observed beneficial effect of multiparity in this cross-breeding. Our results demonstrate that allogeneic pregnancies induce important variations in maternal thymocyte subpopulations depending on the age of the female and the male component of the cross-breeding.

Pro-inflammatory effector Th cells transmigrate through anti-inflammatory environments into the murine fetus

The presence of maternal DNA or even maternal cells within the offspring (microchimerism) has been reported for many fetal tissues, including the liver, heart, and spleen. Microchimerism is believed to be involved in the pathogenesis of autoimmune diseases; however, the cellular origin of this phenomenon remains unknown. Here, we determined whether differentiated T lymphocytes could transmigrate through the immunosuppressive environment of the placenta to reach the fetus. In vitro-differentiated effector/memory Th1 and Th17 cells from OVA₃₂₃₋₃₃₉-specific TCR(tg) T cells of OT-II mice were adoptively transferred (i.v.) into the tail veins of pregnant Ly5.1 mice at d15 and d19 of gestation. Mice were then sacrificed 40 h after adoptive cell transfer. Using radioactive labeling of T cells with sodium chromate [Cr⁵¹] prior to adoptive transfer, we observed that homing of pro-inflammatory Th cells was equally efficient in both pregnant and non-pregnant mice. Transmigration of Th1- and Th17-like cells through the highly immunosuppressive environment of the placenta into the fetus was significantly enhanced in experimental mice compared to control mice (P < 0.0001). In addition, a substantial amount of effector Th cells accumulated in the placenta. Finally, we found that treatment with Pertussis Toxin resulted in a 3-fold increase in the transmigration of effector Th17 cells into the fetus (P < 0.0001). When pro-inflammatory Th1-or Th17-like cells were injected into syngeneic mothers, almost all of the fetuses analyzed exhibited radioactivity, suggesting that transmigration of effector T cells occurs frequently. Our results suggest the possibility of novel roles for these maternal effector cells in the pathogenesis or reduction of disease.

Re-investigation and RNA sequencing-based identification of genes with placenta-specific imprinted expression

Within the vertebrate groups, only mammals are subject to a specialized epigenetic process termed genomic imprinting in which genes are preferentially expressed from one parental allele. Imprinted expression has been reported for >100 mouse genes and, for approximately one-quarter of these genes, the imprinted expression is specific to the placenta (or extraembryonic tissues). This seemingly placenta-specific imprinted expression has garnered much attention, as has the apparent lack of conserved imprinting between the human and mouse placenta. In this study, we used a novel approach to re-investigate the placenta-specific expression using embryo transfer and trophoblast stem cells. We analyzed 20 genes previously reported to show maternal allele-specific expression in the placenta, and only 8 genes were confirmed to be imprinted. Other genes were likely to be falsely identified as imprinted due to their relatively high expression in contaminating maternal cells. Next, we performed a genome-wide transcriptome assay and identified 133 and 955 candidate imprinted genes with paternal allele- and maternal allele-specific expression. Of those we analyzed in detail, 1/6 (Gab1) of the candidates for paternal allele-specific expression and only 1/269 (Ano1) candidates for maternal allele-specific expression were authentically imprinted genes. Imprinting of Ano1 and Gab1 was specific to the placenta and neither gene displayed allele-specific promoter DNA methylation. Imprinting of ANO1, but not GAB1, was conserved in the human placenta. Our findings impose a considerable revision of the current views of placental imprinting.

Vascular endothelial expression of indoleamine 2,3-dioxygenase 1 forms a positive gradient towards the feto-maternal interface

We describe the distribution of indoleamine 2,3-dioxygenase 1 (IDO1) in vascular endothelium of human first-trimester and term placenta. Expression of IDO1 protein on the fetal side of the interface extended from almost exclusively sub-trophoblastic capillaries in first-trimester placenta to a nearly general presence on villous vascular endothelia at term, including also most bigger vessels such as villous arteries and veins of stem villi and vessels of the chorionic plate. Umbilical cord vessels were generally negative for IDO1 protein. In the fetal part of the placenta positivity for IDO1 was restricted to vascular endothelium, which did not co-express HLA-DR. This finding paralleled detectability of IDO1 mRNA in first trimester and term tissue and a high increase in the kynurenine to tryptophan ratio in chorionic villous tissue from first trimester to term placenta. Endothelial cells isolated from the chorionic plate of term placenta expressed IDO1 mRNA in contrast to endothelial cells originating from human umbilical vein, iliac vein or aorta. In first trimester decidua we found endothelium of arteries rather than veins expressing IDO1, which was complementory to expression of HLA-DR. An estimation of IDO activity on the basis of the ratio of kynurenine and tryptophan in blood taken from vessels of the chorionic plate of term placenta indicated far higher values than those found in the peripheral blood of adults. Thus, a gradient of vascular endothelial IDO1 expression is present at both sides of the feto-maternal interface.

Maternal background strain influences fetal-maternal trafficking more than maternal immune competence in mice

The objective of this study was to determine if fetal-maternal cell trafficking is affected by maternal immune competence and/or parental background strain using fluorescence-activated cell sorting (FACS). In our experience the sensitivity of FACS allows for the detection of 5 fetal in 10(7) maternal cells and assessment of cell surface phenotype. Wild-type C57BL/6J (n=18), FVB/NJ (n=15), and immunodeficient B6129S7-Rag1(tm1Mom)/J (n=16) female mice were mated to C57BL/6J males homozygous for the green fluorescent protein (GFP) transgene. Single cell suspensions of maternal lung, liver, spleen, bone marrow, and blood were analyzed between late gestation (day e16-18) and 1 day post-partum for the number of GFP-positive fetal cells in relation to 10(7) maternal cells and the percentage of GFP-positive cells that expressed the surface markers CD11b, CD29, CD34, CD44, or CD105. The highest relative proportions of GFP-positive fetal cells were observed in maternal lungs and livers from immunocompetent allogenic females. Among congenic matings, fetal cell microchimerism was higher in immunodeficient compared with immunocompetent females. Maternal strain and strain differences between the mother and father statistically significantly affected both the numbers of fetal cells and the relative distribution of cell types in maternal organs. The highest relative proportion of fetal cells was observed in allogenic matings with immunocompetent females. Since allogenic matings are more similar to those that occur in humans, future studies using animal models of microchimerism should consider incorporating this type of experimental design.

Pregnancy-associated progenitor cells: an under-recognized potential source of stem cells in maternal lung

Novel therapies are needed for the treatment of acute and chronic lung diseases, many of which are incurable. The use of exogenous stem cells has shown promise in both animal models and clinical trials. However, to date, the stem cell literature has under-recognized naturally acquired pregnancy-associated progenitor cells (PAPCs). These cells are found at sites of injury or disease in female tissues. They persist for decades after parturition in maternal blood and organs, with the largest number being found in the maternal lungs. Their presence there may be one explanation for the sex differences observed in the prevalence and prognosis of some lung diseases. Although the clinical significance of these cells is as yet unknown, the literature suggests that some of the PAPCs are stem cells or have stem cell-like properties. PAPCs harvested from the blood or organs of parous women could potentially be used as an alternate source of cells with regenerative properties for the woman herself or her children. Because PAPCs preferentially traffic to the maternal lung they may play a significant role in recovery or protection from lung disease. In this review article, we discuss ongoing research investigating the administration of both adult and placenta-derived stem cells to treat lung disease, and how PAPCs may also play an important future therapeutic role.

Transplacental traffic after in utero mesenchymal stem cell transplantation

Transplacental traffic of fetal progenitor and differentiated cells is a well-known phenomenon in pregnancies. We hypothesize that intrauterine stem cell transplantation leads to microchimerism in the dams and that this is gestational age-dependent. EGFP+ fetal liver-derived mesenchymal stem cell (MSC) (10(5) per fetus) were injected intraperitoneally into congeneic and allogeneic recipient fetuses at E12 versus E13.5 of murine pregnancy (56 dams). Engraftment in maternal organs was evaluated using TaqMan quantitative polymerase chain reaction (PCR) and fluorescence microscopy during pregnancy (1, 3, and 7 days after in utero transplantation [IUT]) and after delivery (1 and 4 weeks after delivery). One day after IUT donor cells were mainly found in the placenta (E12: 9/10 dams vs. E13.5: 4/8 dams) and laparotomy site (E12: 5/10 dams vs. E13.5: 4/8 dams). Three days after IUT these probabilities decreased significantly in the placenta to 3/8 and 1/3, respectively, whereas it was increased within the surgical wound to 8/8 and 2/4. One week after IUT donor cells could be detected in other single maternal organs, such as bone marrow or spleen. The surgical wound was chimeric in all dams. One week after delivery the surgical wound was still a major site of engraftment in both groups. E12 IUT resulted in detectable donor cell microchimerism in the maternal bone marrow (3/4), liver (2/4), lungs (1/4), spleen (1/4), and thymus (1/4), whereas engraftment probabilities were lower following E13.5 IUT (BM: 1/4, liver: 2/4, lungs: 1/4, spleen: 1/4, thymus: 0/4). At 4 weeks after delivery persistent microchimerism was found only after E12 IUT in various maternal organs (BM: 1/4, spleen: 1/4, lungs: 1/4) and within newly created surgical wounds (3/4), but completely not in the E13.5 group. Allogeneic IUT did also not result in any detectable long-term fetal microchimerism. An earlier IUT might lead to a higher transplacental traffic of donor MSC and persistent microchimerism within maternal tissues. Even 4 weeks after delivery, these cells are present in surgical wounds.

Maternal T cells limit engraftment after in utero hematopoietic cell transplantation in mice

Transplantation of allogeneic stem cells into the early gestational fetus, a treatment termed in utero hematopoietic cell transplantation (IUHCTx), could potentially overcome the limitations of bone marrow transplants, including graft rejection and the chronic immunosuppression required to prevent rejection. However, clinical use of IUHCTx has been hampered by poor engraftment, possibly due to a host immune response against the graft. Since the fetal immune system is relatively immature, we hypothesized that maternal cells trafficking into the fetus may pose the true barrier to effective IUHCTx. Here, we have demonstrated that there is macrochimerism of maternal leukocytes in the blood of unmanipulated mouse fetuses, with substantial increases in T cell trafficking after IUHCTx. To determine the contribution of these maternal lymphocytes to rejection after IUHCTx, we bred T and/or B cell-deficient mothers to wild-type fathers and performed allogeneic IUHCTx into the immunocompetent fetuses. There was a marked improvement in engraftment if the mother lacked T cells but not B cells, indicating that maternal T cells are the main barrier to engraftment. Furthermore, when the graft was matched to the mother, there was no difference in engraftment between syngeneic and allogeneic fetal recipients. Our study suggests that the clinical success of IUHCTx may be improved by transplanting cells matched to the mother.

Transplantation tolerance to a single noninherited MHC class I maternal alloantigen studied in a TCR-transgenic mouse model

The mechanisms underlying tolerance to noninherited maternal Ags (NIMA) are not fully understood. In this study, we designed a double-transgenic model in which all the offspring's CD8(+) T cells corresponded to a single clone recognizing the K(b) MHC class I protein. In contrast, the mother and the father of the offspring differed by the expression of a single Ag, K(b), that served as NIMA. We investigated the influence of NIMA exposure on the offspring thymic T cell selection during ontogeny and on its peripheral T cell response during adulthood. We observed that anti-K(b) thymocytes were exposed to NIMA and became activated during fetal life but were not deleted. Strikingly, adult mice exposed to NIMA accepted permanently K(b+) heart allografts despite the presence of normal levels of anti-K(b) TCR transgenic T cells. Transplant tolerance was associated with a lack of a proinflammatory alloreactive T cell response and an activation/expansion of T cells producing IL-4 and IL-10. In addition, we observed that tolerance to NIMA K(b) was abrogated via depletion of CD4(+) but not CD8(+) T cells and could be transferred to naive nonexposed mice via adoptive transfer of CD4(+)CD25(high) T cell expressing Foxp3 isolated from NIMA mice.

Identification and resolution of artifacts in the interpretation of imprinted gene expression

Genomic imprinting refers to genes that are epigenetically programmed in the germline to express exclusively or preferentially one allele in a parent-of-origin manner. Expression-based genome-wide screening for the identification of imprinted genes has failed to uncover a significant number of new imprinted genes, probably because of the high tissue- and developmental-stage specificity of imprinted gene expression. A very large number of technical and biological artifacts can also lead to the erroneous evidence of imprinted gene expression. In this article, we focus on three common sources of potential confounding effects: (i) random monoallelic expression in monoclonal cell populations, (ii) genetically determined monoallelic expression and (iii) contamination or infiltration of embryonic tissues with maternal material. This last situation specifically applies to genes that occur as maternally expressed in the placenta. Beside the use of reciprocal crosses that are instrumental to confirm the parental specificity of expression, we provide additional methods for the detection and elimination of these situations that can be misinterpreted as cases of imprinted expression.

Increased fetal cell trafficking in murine lung following complete pregnancy loss from exposure to lipopolysaccharide

To determine whether chemically induced miscarriage affects fetomaternal trafficking in a mouse model, we measured the amount of fetal DNA present in various maternal organs by polymerase chain reaction amplification following exposure to lipopolysaccharide (LPS). As the frequency of fetal cells and the number of animals with detectable microchimerism following LPS injection were significantly increased, particularly in lung tissue compared to controls, with no signs of an inflammatory response, we conclude that LPS-induced miscarriage results in increased murine fetomaternal cell trafficking, supporting a relationship between fetal loss and the establishment of fetal cell microchimerism.

CD34+ cells in maternal placental blood are mainly fetal in origin and express endothelial markers

Fetal CD34+ cells enter the maternal circulation during pregnancy and may persist for decades. These cells are usually depicted as hematopoietic stem/progenitor cells. Our objective was to further determine the phenotype of fetal chimeric CD34+ cells in placental maternal blood from the intervillous space (IVS). Human healthy term placentas were analyzed (n=9). All fetuses were male. CD34+ cells were identified in the IVS and further characterized as fetal or maternal using X and Y chromosome fluorescence in situ hybridization. The phenotype of fetal cells was further analyzed using anti-CD117 (c-kit), anti-CD133, anti-CD31, anti-von Willebrand factor (vWF), anti-vimentin, anti-CD45 and anti-cytokeratin (CK) antibodies. We used preeclamptic placentas of male (n=3) and healthy placentas of female fetuses (n=3) as controls. As expected fetal cells were easily identified in the IVS and significantly increased in cases of preeclampsia. Most CD34+ cells in the IVS were of fetal origin (90%) and were not surrounded by CK staining further showing that they were not in fetal trophoblastic villi. Similarly, about 40% of CD31+ and 6% of vimentin+ cells in the IVS were fetal in origin. No CD117+ or CD133+ fetal cells were found in the IVS of examined placentas. Besides, all the CD34+ cells identified in the IVS were co-labeled with vWF or CD31, suggesting their endothelial origin. These results suggest that most CD34+ cells in maternal placental blood at term are fetal in origin from endothelial and not hematopoietic lineages.

Fetomaternal trafficking in the mouse increases as delivery approaches and is highest in the maternal lung

The purpose of the study was to understand in more detail the natural history of fetomaternal cell trafficking in healthy pregnant mice. Our goal was to identify the best target organs and days during pregnancy for further mechanistic studies of the role of fetal cells in maternal disease and injury. C57BL/6J wild-type virgin females (n = 54) were mated with congenic enhanced green fluorescent protein (EGFP) transgenic males. During pregnancy and after delivery, female mice were euthanized, and eight organs and blood were analyzed for the presence of fetal GFP+ cells with flow cytometry and real-time quantitative PCR. Maternal lungs, liver, and spleen were also analyzed by fluorescent stereomicroscopy. Fetal GFP+ cells were first found at low frequency at Embryonic Day 11, increased to a maximum at Embryonic Day 19, and decreased rapidly postpartum. These fetal cell dynamics were significantly reproducible among all mice studied. In addition, there was a consistent distribution of fetal cells within maternal organs, with lung, liver, blood, and spleen having the greatest concentrations; these were highly correlated at all time points (P < 0.0001). Maternal lung contained 10- to 100-fold more fetal cells than any other organ, and using all three techniques, the number of fetal cells detected was the most consistent and reproducible in this organ. Stereomicroscopy showed that within the lung, fetal cells were widely and apparently randomly distributed. Using a murine model, our data demonstrate that fetomaternal cellular trafficking occurs in reproducible patterns, is maximal near term delivery, and has predilection for the maternal lung.

Heredity--venturing beyond genetics

Our knowledge of heredity has recently undergone major upheaval. Heredity transmits considerably more than just genetic elements. First, the oocyte is full of maternal cytoplasmic components that subsequently are present in each new cell. Second, maternal cells can pass to the progeny, where they remain active into adult life (microchimerism). Here, we examine the notion that the transmission of characters involves at least two processes in addition to that of mendelian heredity, long considered to be the only hereditary mechanism. These processes all involve epigenetic processes, including the transmission of macromolecules, subcellular organelles, and living cells solely from the mother to her offspring, whether female or male, during pregnancy and lactation. We postulate that cytoplasmic heredity and maternal transmission of cells leading to a long-term state of microchimerism in progeny are two good examples of matrilineal, nonmendelian heredity. A mother's important contribution to the development and health of her progeny seems to possess many uncharted depths.

Murine maternal cell microchimerism: analysis using real-time PCR and in vivo imaging

In humans, maternal cells are present in the affected tissues of children with inflammatory myopathy, scleroderma, and neonatal lupus. It is unknown if maternal cell microchimerism (MCM) contributes to the pathology of disease. We sought to understand the factors that affect MCM to serve as a baseline for future mechanistic studies. Using a mouse model, we bred female mice transgenic for the luciferase (Luc) reporter gene to wild-type (WT) males. The WT offspring were sacrificed at various postnatal ages. DNA was extracted from multiple organs, and real-time PCR amplification was used to quantify Luc transgene as a marker for maternally derived cells. Sensitivity was one to two transgenic cells per 100,000 WT cells. MCM was noted in 85% of mice and 45% of tissues assayed. The average quantity of MCM was 158 maternal cells per 100,000 neonatal cells. The organs displaying the highest frequency and quantity of MCM were heart and lung (P < 0.001). Postnatal age up to 21 days did not appear to affect levels of MCM (P = 0.47), whereas increasing parity may increase levels of MCM. The data show that MCM is a common occurrence in healthy newborn mice, that it is present in their major organs, and that there are organ specific differences. This may represent differential migration of maternal cells or varying receptivity of specific fetal organs to microchimerism. Pregnancy history appears to play a role in maternal cell trafficking. The role of MCM in pregnancy and disease pathogenesis remains to be elucidated.